Summary:
FocA is a pH dependent, bidirectional formate transporter with an important role in regulating intracellular formate levels during anaerobic respiration and mixed acid fermentation in E.coli K-12. Formate, produced by the action of pyruvate:formate lyase is exported from the cytoplasm where, in the presence of terminal electron acceptors, it is a substrate of the periplasmic formate dehydrogenases. In the absence of terminal electron acceptors it is reimported to the cytoplasm for use by the formate:hydrogen lyase complex (reviewed in [Sawers05])

Analysis of formate levels in fermenting E.coli cultures indicate that initially, formate is exported out of the cell in order to prevent acidification of the cytoplasm. focA knock-out mutants grown in this manner are associated with increased intracellular levels of formate and decreased excretion of formate. However, if the pH of the culture medium drops below about 6.8, formate is re-imported and FocA has also been implicated in this process [Suppmann94]. FocA is required for the import of the toxic formate analogue, hypophosphite [Suppmann94].

The FocA compex is a symmetric pentamer forming a single central pore believed to be occupied by lipid molecules, plus 5 axial pores (one per protomer) that may be the site of transport [Wang09, Falke09]. Each protomer contains six transmembrane helices [Suppmann94, Wang09]. Both the amino and carboxy termini of FocA protomers are located on the cytoplasmic side of the inner membrane [Wang09].

The energetics of formate transport are unclear although structural evidence suggests that FocA functions as a channel rather than a transporter [Wang09]. The FocA complex must undergo some sort of pH dependent functional switch [Suppmann94]. Molecular dynamics simulations suggest that the axial channel size is affected by the pH environment [Feng12].

The focA gene is encoded in an anaerobically-induced operon with pflA and pflB, encoding pyruvate-formate lyase [Sawers89, Suppmann94]. E. coli is also likely to have a second formate transporter [Suppmann94].

FocA belongs to the formate-nitrite transporter (FNT) family [Saier99]

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History:
Peter D. Karp on Wed Jan 18, 2006:
Gene right-end position adjusted based on analysis performed in the 2005 E.
coli annotation update [Riley06
].
Socorro Gama-Castro on Fri Oct 29, 2004:
The start site of this gene was originally assigned solely on the basis of sequence
considerations [Blattner97
] . However, it was
changed because Sawers and Bock [Sawers89
] attested that the real
start site is actually located 183 bp downstream. The demonstration is based on
identification of both a plausible ribosome binding site and three promoters at an
appropriate distance from the new start site. The promoters were identified using primer
extension analysis.
10/20/97 Gene b0904 from Blattner lab Genbank (v. M52) entry merged into EcoCyc gene
EG11258; confirmed by SwissProt match.